A variety of orthopedic and neurological procedures make use of fasteners in constructs connecting one bone, or bone fragment, to another. For example, the connection of one vertebra of the human spine to another vertebra is a common beneficial procedure. The vertebrae of the human spine are arranged in a column with one vertebra on top of the next. An intervertebral disc lies between adjacent vertebrae to transmit force between the adjacent vertebrae and provide a cushion between them. The discs allow the spine to flex and twist.
With age, spinal discs begin to break down, or degenerate resulting in the loss, of fluid in the discs and consequently resulting in them becoming less flexible. Likewise, the disks become thinner allowing the vertebrae to move closer together. Degeneration may also result in tears or cracks in the outer layer, or annulus, of the disc. The disc may begin to bulge outwardly. In more severe cases, the inner material of the disc, or nucleus, may actually extrude out of the disc.
In a process known as spinal stenosis, the spinal canal may narrow due to excessive bone growth, thickening of tissue in the canal (such as ligamentous material), or both.
The facet joints between adjacent vertebrae may degenerate and cause localized and/or radiating pain.
In addition to degenerative changes in the disc, the spine may undergo changes due to trauma from automobile accidents, falls, heavy lifting, and other activities.
The spine may also be malformed from birth or become malformed over time such as for example in cases of scoliosis, kyphosis, spondylosis, spondylolisthesis, and other deformities.
The conditions described above can result in disfigurement, pain, numbness, weakness, or even paralysis in various parts of the body. All of the above conditions and similar conditions are collectively, referred to herein as spine disease.
Typically, surgeons treat spine disease by attempting to stabilize adjacent vertebrae relative to one another and/or restore the normal, spacing between adjacent vertebrae to improve the shape of the spine and to relieve pressure on affected nerve tissue. Stabilizing the vertebrae is often accomplished with plates and/or rods attached to the vertebrae with fasteners such as screws such as for example pedicle screws. The stabilization may be rigid such that it eliminates motion between adjacent vertebrae and encourages bony fusion between the vertebrae or it may be dynamic to allow continued motion between the vertebrae. Often, the stabilization includes inserting a rigid spacer made of bone, metal, or plastic into the disc space between the adjacent vertebrae and allowing the vertebrae to grow together, or fuse, into a single piece of bone.
The patients anatomy and/or the desired correction frequently require aligning the fastener at various angles relative to the bone and the rest of the stabilizing construct. Screws have been developed that are able to be angled relative to the stabilizing construct and they are typically referred to as “polyaxial” screws. However, despite the fact that numerous such polyaxial screw systems have been marketed, improvements are desirable. In particular, current devices provide limited angular adjustment.